1. Field of the Invention
The present invention relates generally to wide-area intruder detection and tracking networks, and more particularly, to the use of a plurality of RF waveguide sensors for detecting and tracking objects, including people, within a wide-area.
2. Prior Art
In recent years, numerous sensors and sensory systems have been developed to detect and track intruders. Many of these sensors have found applications in safety, homeland security and other similar civilian and military areas. For sensors used in such applications they must be made to be relatively small and be assembled in small packaging. The sensors must also require low power, be capable of remote operation, and may be required to be capable of one- or two-way communication with a central station or networked using some wireless technology.
To monitor a wide area for intrusion and tracking of the intruders, a network of appropriate sensors is required. To make such a network practical, it is highly desirable for all the nodes except a few where the data is collected and analyzed to be passive. This is in fact necessary if the network is to be deployed from a distance into hard to reach areas, areas that are contaminated by chemical or biological or other similar hazardous materials or an area suspected of being occupied by hostile elements such as terrorists. In any case, it is preferable that the nodes, even those that are active, have low power requirements.
There are several types of sensors known in the art for detection and tracking of objects, including acoustic, visual, RF sensors and radar systems. Some factors that need to be considered for the detection and tracking of objects are the purpose of the network, ease of access to the nodes, available methods of powering each node (if any), physical size and weight of the sensors, the complexity of the data received (which determines the amount of signal processing that needs to be performed prior to transmission of the data), signal-to-noise ratio that can be tolerated, the requirements for sensor (node) concealment, and the requirements for signal stealthness and/or security.
Acoustic sensors can be considered in two categories—active or passive sensors. Microphones serve as passive acoustic sensors in that acoustic signals are received at the sensor without signals being transmitted from the sensor. While passive acoustic sensors offer the advantage of low power consumption, their sensing range is very short and the signal-to-noise ratio of these sensors is relatively poor. Active sensors, such as ultrasonic sensors, include a transmitting and receiving capability. Active acoustic sensors offer the advantage of locating the position of an object simply from a single sensor (location of objects can be achieved with passive acoustic sensors but at least two sensors are required for triangulation). However, active acoustic sensors draw significant power to pulse the acoustic membrane to transmit the ultrasonic signal and their range is too short to cover distances measured in kilometers rather than tens of meters.
Visual sensors offer better signal-to-noise ratio than acoustic sensors. The field of view and range of visual sensors depends on the optical system interfacing the observed scene to the pixel array of visual sensing elements. Visual information can be pre-processed on the sensor using pattern recognition algorithms and only if the processed data indicates a change in the scene, the visual information may be transmitted. Visual sensors may be used in the visible or infrared part of the electromagnetic spectrum. Each has advantages and disadvantages. Visible sensors offer higher signal-to-noise ratio than infrared sensors during the day but are poorer at night. Both sensors can be made to operate at moderately low power. The data processing, however, may require relatively fast processor speed and therefore consume significant power since real time signal processing is required for the present application. Tracking of objects can be relatively easily achieved in optical systems if the fields of view of adjacent sensors overlap since as an object moves from one sensor field of view to another, a hand off communication protocol can be used to pass the information along the sensor network. Visual sensors also have relatively short range for use with a wide-area intruder detection and tracking network where hundreds of square miles needs to be monitored with relatively few nodes. Visual sensors also only work for line-of-site detection and are relatively large and delicate.
Radar systems are presently widely deployed for detection and tracking systems. The size of the antenna and the frequency of the RF signal govern the precision and accuracy with which objects are located. Atmospheric attenuation is large at high frequencies and so low frequency radar is generally required, which requires a large physical size for the antenna. Scanning of scenes can be performed by either mechanically rotating the antenna or by modulating the signal phase to individual elements in an antenna array (smart antennas). The physical size and power requirements for radar systems precludes their use in sensor networks for monitoring and tracking of objects in border security systems envisaged in the proposed network of sensors.
Networks of sensors require communication between the individual nodes. Significant work has been done developing communication protocols for such networks to minimize power consumption, allow for establishing relative locations of sensor nodes in ad hoc networks, and ensuring network continuity in the event of loss of a single node (or multiple nodes) in the network.